7.2 Equations of Straight Lines

7.2 Equations of Straight Lines

Inclination of a Line: The angle $\alpha$ ($0^\circ < \alpha < 180^\circ$) is measured counterclockwise from positive $x$-axis to a non-horizontal straight line $\ell$ is called the inclination of $\ell$.

[Diagram showing inclination of line]

Observe that the angle $\alpha$ in the different positions of the line $\ell$ is $\alpha, 0^\circ$ and $90^\circ$ respectively.

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Note:
(i) If $\ell$ is parallel to $x$-axis, then $\alpha = 0^\circ$
(ii) If $\ell$ is parallel to $y$-axis, then $\alpha = 90^\circ$